Synergism and photocatalytic water splitting to hydrogen over M/TiO2 catalysts: Effect of initial particle size of TiO2

Abstract

In order to study the effect of anatase/rutile phases of TiO2 on the photo-catalytic production of hydrogen (often invoked as synergism) two series of Pt/TiO2 materials were prepared. The initial phase of TiO2 in both series is anatase but their particle size is different. In one case the mean particle size of TiO2 is ca. 15–20nm (nano) and in the other it is ca. 100nm (micro). Before the deposition of Pt, the two semiconductors (nano and micro) were heated to elevated temperatures to obtain partial (and total) transformation of the anatase phase to the rutile phase (UV–vis, XRD, XPS-valence band). On this differently prepared mixed phases Pt was deposited (ca. 1at.%; corrected XPS Pt4f/Ti2p=ca. 0.05) and each series was tested for the photocatalytic production of hydrogen from water in presence of ethanol (5vol.%) as a sacrificial agent (under identical conditions). Based on rates per unit mass no synergism for hydrogen production was seen in both cases; this is in part due to the decrease in the BET surface area during the phase transformation. However, strong synergism was observed for hydrogen production rates per unit area when the initial starting semiconductor was anatase nano (while no synergism seen in the case of the micro crystals). Maximum rate observed was in the 10–30% rutile range. Possible reasons for these differences are discussed.